1. Field of the Invention
The present invention relates to a surface-mounting type antenna, an antenna device, and a communication device including the antenna device. More particularly, the present invention relates to a surface-mounting type antenna, an antenna device, and a communication device including the antenna device which are to be used for mobile communication, etc.
2. Description of the Related Art
While the reduction in size and weight of mobile communication devices, particularly portable telephones in recent years is in progress, as for the antennas mounted on them the further reduction in size and weight, and increase in gain have been required.
In FIGS. 9 and 10, a conventional surface-mounting type antenna and an antenna device including the antenna device are shown respectively. The structure of the surface-mounting type antenna 30 in FIG. 9 is shown in Japanese Unexamined Patent Publication No. 10-13139.
In FIG. 9, the surface-mounting type antenna 10 is composed of some electrodes disposed on the surface of a base member 11 in the form of a rectangular solid made of a dielectric substance such as ceramics, resin, etc. as one insulating material. First, on the nearly whole surface of a first major surface 11a of the base member 11 the grounding electrode 12 is disposed. Further, on a second major surface 11b of the base member 11 a strip-like radiation electrode 13 is disposed along the long side of the base member 11. At a first end of the radiation electrode 13 an open-ended terminal 13a is provided, and ta second end is connected to a grounding electrode through a connecting electrode 14 disposed on an side surface 11c of the base member 11. Further, on the second major surface 11b of the base member 11 a power-supply electrode 15 located close to the open-ended terminal 13a of the radiation electrode 13 is disposed, and the power-supply electrode 15 is connected to a power-supply terminal 16 disposed over an side surface 11d to the first major surface 11a of the base member 11.
Here, when the surface-mounting type antenna 10 is mounted on a circuit board (not illustrated) because a power-supply terminal 16 is connected to a power-supply line on the side of the circuit board by soldering, etc., it is called the terminal in order to distinguish that from other electrodes. Hereinafter, when an electrode is described as a terminal, the electrode for connection to a circuit board is meant. However, there are cases in which electrodes and terminals are integrated, and then a part of the electrodes may be used as a terminal.
Next, in the antenna device 1 shown in FIG. 10, the surface-mounting type antenna 10 is mounted on the circuit-board grounding electrode 3 in the vicinity of a corner portion of the circuit board 2. The grounding electrode 12 and power-supply terminal 16 of the surface-mounting type antenna 10 are connected to the circuit-board grounding electrode 3 and power-supply line 4 disposed on the circuit board 2 by soldering, etc. respectively.
Here, in FIG. 11, an equivalent circuit of the antenna device 1 in FIG. 10 is shown. In FIG. 11, a capacitor CO represents capacitance produced between the power-supply electrode 15 and the grounding electrode 12 and circuit-board grounding electrode 3, a capacitor C1 capacitance between the power-supply electrode 15 and the open-ended terminal 13a of the radiation electrode 13, a capacitor C2 capacitance between the radiation electrode 13 and the grounding electrode 12 and circuit-board grounding electrode 3, conductance G a radiation resistor of the surface-mounting type antenna 10, and an inductance L1 and resistor R1 an inductance component and resistance component of the radiation electrode 13 respectively. Further, mark S represents a signal source. The inductance L and resistor R1 are connected in series, and one end of such is connected to the signal source S through the capacitor C1 and the other end is grounded. The connecting portion between the inductance L1 and capacitor C1 is grounded through the capacitor C2 and through the conductance G respectively. More, the connecting portion between the capacitor C1 and signal source S is grounded through the capacitor C0. And the resonance frequency of the antenna device 1 is determined mainly by the inductance L1 and capacitor C2.
Further, in FIGS. 12 and 13, another conventional surface-mounting type antenna and antenna device including the antenna device are shown. In FIG. 13, to the same or equivalent portions as in FIG. 10, the same reference numerals are given and their explanation is omitted. The structure of the surface-mounting type antenna 20 in FIG. 12 is shown in Japanese Unexamined Patent Publication No. 10-13139.
In FIG. 12, the surface-mounting type antenna 30 is composed of some electrodes disposed on the surface of a base member 31 in the form of a rectangular solid made up of a dielectric substance such as ceramics, resin, etc. as one insulating material. First of all, a strip-like radiation electrode 32 is disposed along the long side of the side surface 31c and over the second major surface 31b of the base member 31. A first end of the radiation electrode 32 is served as an open-ended terminal on the second major surface 31b of the base member 31, and a second end is connected to the grounding terminal 33 disposed on the first major surface 31a of the base member 31. Further, a power-supply electrode 34 is disposed on the second major surface 31b of the base member 31, and the power-supply electrode 34 is connected to a power-supply terminal 35 disposed over the side surface 31d to the first major surface 31a of the base member 31. In the same way, on the second major surface 31b of the base member 31, a grounding electrode 36 is disposed in the vicinity of the open-ended terminal 32a of the radiation electrode 32, and the grounding electrode 36 is connected to a grounding terminal 37 disposed over the side surface 31d to the first major surface 31a of the base member 31.
Next, in an antenna device 20 shown in FIG. 13, the surface-mounting type antenna 30 is mounted in an area 2a having no electrode disposed in the vicinity of a corner portion of the circuit board 2. The grounding terminals 33 and 37 and power-supply terminal 35 of the surface-mounting type antenna 30 are connected to the circuit-board grounding electrode 3 and power-supply line 4 respectively by soldering, etc.
Further, in an equivalent circuit of the antenna device 20, the capacitor C2 mainly represents a capacitance produced between the open-ended terminal 32a and the grounding electrode 36, grounding terminal 37, and circuit-board grounding electrodes of the radiation electrode 32, and the equivalent circuit is basically the same as in FIG. 11. Accordingly, here the explanation is omitted.
In order to realize the reduction in size of a communication device equipped with a surface-mounting type antenna, it is necessary to reduce the space occupied by the antenna device on the circuit board, and as a method for the reduction, the reduction in size of the surface-mounting type antenna itself is one choice to be considered.
In the surface-mounting type antennas shown in FIGS. 9 and 12, if the base member is simply made small, the length of the radiation electrode is reduced and as a result the inductance L1 of the radiation electrode is also reduced. Because of this, in order to realize the same inductance L1 on the equivalent circuit as before, it is necessary to make the radiation electrode thin or have the radiation electrode formed in a meandering way. However, in that case, there is a problem that the resistance component R1 of the radiation electrode is increased and the antenna gain is reduced. To the contrary, it may be considered that the increase of capacitance C2 compensates for the reduction of inductance L1 in order to keep the same resonance frequency, but for that purpose it is necessary to increase the dielectric constant of the base member and make the space between the open-ended terminal of the radiation electrode and the grounding electrode narrow and then there is a problem that because the radiation resistance G is increased, the antenna gain is reduced and the bandwidth is narrowed. As a result, in the communication device equipped with such an antenna device there occurs a problem that the antenna gain is lowered and the bandwidth is made narrow.